As robots have gained importance in the field of manufacturing processes, so in parallel greater automation has been achieved thanks to new technologies. At the beginning of the robotic era, robotic grippers were used in straightforward manufacturing tasks such as for car assembling tasks. However, today robotic grippers must do more than just grasp a same item or similar items repeatedly. Indeed, robotic grippers are expected to be capable of handling complex objects that may have a variety of different shapes, or be made of unstructured fabrics or fragile materials. Robots are now expected to have some “sense” of how to accomplish a manufacturing task and successfully handle a variety of objects without damaging them.
The human hand, with its various mechanoreceptors, remains the best-functioning “device” for object-manipulation tasks. In an attempt to replicate these functions robotically, researchers have developed tactile sensors based on numerous different sensing principles, such as by using piezoresistive rubber, conductive ink, piezoelectric material, conductive fluid, and measuring a change in capacitance. Most of these approaches are about measuring a contact pressure. However, the human sense of touch does not rely on contact pressure alone. It also uses vibration, temperature, and shear loading, among others. These additional modalities let humans recognize surface texture, detect object slippage, and perceive other complex events. With this in mind, some researchers in robotics are now building multimodal tactile sensors in hopes of giving robots a sense of touch that is more similar to the human one. Along with detecting pressure localization and magnitude, these modern sensors can also detect contact events like vibration. For example, some have developed a variable resistor ink sensor that can also detect incipient slip thanks to the use of Polyvinylidene fluoride (PVDF). Others have developed a multimodal sensor for fabric manipulation and classification.
One well-known multimodal sensor is the commercially-available BioTac™ tactile sensor provided by SynTouch LLC and described in U.S. Pat. Nos. 7,658,119, 8,181,540, and 8,272,278 to Loeb et al. The BioTac™ tactile sensor can measure vibrations in addition to temperature and pressure. The tactile sensor has a conductive plate with multiple electrode points arranged in a two-dimensional array such as presented in Prior Art FIG. 1. Each electrode point 100 is connected to an impedance measuring circuitry 102, and is surrounded by a weakly conductive fluid or pulp contained within an elastomeric skin. When an external force is applied to the skin, a variation in the fluid paths around the electrode points produces a distributed pattern of impedance changes indicative of information about the forces and objects that applied them. In one example, the impedance measuring circuitry is configured to detect changes in the electrical impedance of the volume conductive liquid between the electrodes, and to interpret such changes under certain circumstances as being indicative of a shear force that is applied to the skin. The tactile sensor is thereby able to measure micro-vibrations due to sliding friction, as well as to measure pressure. A same electrode 100 is used for providing a micro-vibration measurement and a pressure measurement. The electrodes are alternatingly connected to a vibration sensing subsystem and to a pressure sensing subsystem. A multiplexer 104 selects each electrode in turn for connecting to one of the vibration sensing 106 or pressure sensing 108 measuring circuitries, according to instructions received by a microcontroller 110.
As can be noticed from these sensors, sensors for grasping applications need to be capable of more than simply the ability to sense forces. However, the aforementioned sensors require the use of special materials and complex structures that can be difficult to assemble, fabricate, and maintain. In particular, the Bio Tac™ sensor requires a specialized technician to inject a fluid under the skin, which can result in some downtime since the skin of the sensor can wear out frequently. Moreover, the BioTac™ sensor requires a whole phalange to be replaced in order for it to be integrated with a robotic hand.
Other solutions have been developed to provide multimodal sensing. One solution relates to capacitive sensing. Capacitive sensors appear to be a suitable candidate for multimodal tactile sensing due to their simplicity and easy-to-implement properties. The performance of a capacitive sensor depends on its electrical circuit and the electro-mechanical characteristics of its dielectric. Researchers have developed capacitive sensors that can perform both static and dynamic sensing by using integrated circuits (ICs) that enable the sensor's electronic circuit to process the additional data needed for dynamic sensing. As a result, such sensors are capable of classifying various types of contact events.
It has been shown that by cleverly designing the dielectric, the sensor's sensitivity can be greatly enhanced. Several researchers have succeeded in improving the sensitivity of their capacitive sensors by using dielectrics made of elastomer foam and microstructured rubber. Another research group attained extremely high sensitivity using a microstructured dielectric made of nanoparticle-filled elastomer, such as presented in Prior Art FIG. 2A. The sensor 200 has a pair of spaced apart conductive plates 202 with a dielectric 204 there between. The dielectric 204 has a microstructure of a plurality of protrusions 206 conductively extending between the two conductive plates 202. Each protrusion 206 has at least two layers 208 and 210. The first layer 208 having a greater diameter than that of the second layer 210 accounts for significant variations between the two plates 202 and reacts to greater pressure ranges. The second layer 210 accounts for weaker variations between the two plates 202 and reacts to lower pressure ranges. However, these highly sensitive capacitive sensors are inconvenient and time-consuming to manufacture due to the specialized dielectric fabrication processes.
In US Patent Publication No. 2015/0355039 to Duchaine et al. there is presented a method of using invert molding to cast the dielectric out of liquid elastomer filled with nanoparticles. The casted dielectric has a dielectric constant of 12. Prior art FIG. 2B depicts the various steps in manufacturing the dielectric. A mold is first provided and is filled with liquid elastomer filler. A conductive fabric is then placed in contact with the filler before curing. Once cured, the combination of the molded dielectric and the conductive fabric is removed from the mold. Prior art FIG. 2C presents a magnified view of the molded dielectric 204. As can be noticed, each protrusion 206 of the molded dielectric 204 has a different shape, and the dielectric does not provide a consistent thickness. Moreover, the method is very time-consuming due to the invert molding process and cannot be applied in mass production of sensors, since the molding process can take several days.